In the 1960s, practitioners in the field of microelectronics first developed techniques for fabricating tiny mechanical structures using a series of steps involving the depositing of layers of materials onto the surface of a silicon wafer substrate, followed by selectively etching away parts of the deposited materials. By the 1980s, the industry began moving toward silicon-based surface micromachining using polysilicon as the mechanical layer. However, although polysilicon has proven a useful building block in fabricating microelectromechanical systems (MEMS) because of its mechanical, electrical, and thermal properties, fabrication techniques used for polysilicon-based MEMS do not work well with fabrication techniques used for complementary metal-oxide semiconductor (CMOS) technology. As such, in the prior art, the circuitry for controlling the MEMS traditionally was fabricated on a separate die. While there has been some success in integrating CMOS and polysilicon fabrication on a single die, these hybrid CMOS-polysilicon devices have proven less than ideal because of long design times and complex fabrication requirements.
More recently, practitioners have attempted to fabricate MEMS structures using standard CMOS materials rather than the materials traditionally used in polysilicon-based MEMS structures. In standard CMOS fabrication, transistors are formed on the surface of a silicon wafer and electrical pathways are built above the transistors by repeatedly depositing and selectively removing layers of metallic and dielectric material. In an integrated CMOS/MEMS die, at the same time as the CMOS circuits are being interconnected on one part of the wafer, patterned layers of metallic and dielectric materials on another part of the wafer can form complex MEMS structures. Once all of the layers have been built up, the MEMS structure is “released”—that is, the sacrificial dielectric material around the MEMS structures is removed using an etchant such as vHF (vapor hydrofluoric acid), leaving the mechanical components of the MEMS structure free to move. Other sacrificial etchants can be used such as a wet “pad etch,” plasma or RIE dry etching, or a combination of any of these. Certain sacrificial etchants attack the silicon nitride passivation. Polyimide, included in some CMOS processes on top of the passivation layer can mitigate the attack on the silicon nitride.
This simplifies the design and manufacturing since there is no need for the use of special procedures and materials to accommodate the disparate requirements of fabricating a hybrid CMOS-polysilicon die. However, as a structural building block, the metallic layers used in CMOS lack the stiffness required for use as structural MEMS components, and moreover, the thin metallic layers tend to curve after release. While it is possible to address these problems by building structures composed of stacked layers of metal having with metal vias connecting each metallic layer, many other problems remain unresolved.
First, while a multi-layer metallic MEMS structure may be rigid, in some instances the rigidity of a MEMS structure should be anisotropic (that is, rigid in one axis of movement and flexible in another axis of movement). For example, many MEMS structures use springs to control movement; using multiple layers of metal for a spring structure may create the extra stiffness that prevents the spring from curving, but the stiffness in the x-, y-, and z-axes may limit the structure's effectiveness as a spring.
Second, many types of MEMS require an airtight chamber after release, so either a cap wafer must be installed or else holes must be created in the top layer to allow the etchant to reach the dielectric material. In the former case, attaching a cap wafer requires non-standard CMOS processing and cost, makes access to the bonding pads more challenging, and adds height to the die. In the latter case, in order to seal the holes after the etching step, metal or other materials must be deposited, which risks inadvertent introduction of the sealing material into the interior of the chamber, potentially affecting the movement of the mechanical components.
Third, in order to remove the dielectric material, the vHF (or other sacrificial etchants) must come into physical contact with the material. For a narrow stacked structure, the vHF can readily remove the dielectric material. However, for a wide plate structure (for example, a microphone back plate), the vHF may take considerable time to reach the interior of the plate, and this may result in removal of more dielectric material than desired from other parts of the MEMS structure.
Fourth, for a wide plate structure, even after removal of the dielectric material between the metallic layers, the plate may have significant mass. This can lead to lower resonant frequencies, which can negatively impact the frequency response of the microphone.
Fifth, as noted above, single layers of metal are relatively weak. Where an unreinforced top metallic layer covers a sealed chamber containing the MEMS structure, the top layer may bow inward because of the vacuum within the chamber. Adding space between the MEMS structure and the top layer may keep the top layer from interfering with the MEMS structure, but the additional space increases the height of the die.
Sixth, when the surfaces of mechanical components of a MEMS structure come into contact with one another, adhesive surface forces, commonly known as “stiction,” can cause the surfaces to become stuck to one another, compromising the mechanical functions of the device.
Therefore, there is an unmet need for structures and methods that address the known problems in fabricating integrated CMOS/MEMS dice.